connecting two nonadjacent angles or vertices of a polygon or polyhedron, as a straight line.
extending from one edge of a solid figure to an opposite edge, as a plane.
having an oblique direction.
having oblique lines, ridges, markings, etc.
noun
a diagonal line or plane.
virgule.
a diagonal row, part, pattern, etc.
Manège. (of a horse at a trot) the foreleg and the hind leg, diagonally opposite, which move forward simultaneously.
diagonal cloth.
Mathematics. a set of entries in a square matrix running either from upper left to lower right (main diagonal, or principal diagonal ) or lower left to upper right (secondary diagonal ).
Chess. one of the oblique lines of squares on a chessboard: He advanced his bishop along the open diagonal.
Origin of diagonal
First recorded in 1535–45; from Latin diagōnālis, from Greek diagṓn(ios) “from angle to angle” + Latin -ālis adjective suffix; see dia-, -gon, -al1